• Lars T. Kyllingstad (9/9) Jul 14 2010 Is object.reserve() only useful for large arrays, or should I always use...
• Steven Schveighoffer (18/26) Jul 14 2010 Yes, you should always reserve if you know how much data is coming. If ...
• Lars T. Kyllingstad (5/38) Jul 14 2010 Yeah, that was just an artificial example. :) The actual use case was

"Lars T. Kyllingstad" <public kyllingen.NOSPAMnet> writes:

Is object.reserve() only useful for large arrays, or should I always use 
it if I intend to append to an array?

Let's say I want to read some data, and I expect there to be ~100 bytes 
of data.  Is there any point in using reserve() first, or will there 
always be that much memory available to an array?

  byte[] buffer;
  buffer.reserve(100);
  foreach(byte b; dataSource) buffer ~= b;

-Lars

"Steven Schveighoffer" <schveiguy yahoo.com> writes:

On Wed, 14 Jul 2010 06:01:10 -0400, Lars T. Kyllingstad  
<public kyllingen.nospamnet> wrote:

 Is object.reserve() only useful for large arrays, or should I always use
 it if I intend to append to an array?

 Let's say I want to read some data, and I expect there to be ~100 bytes
 of data.  Is there any point in using reserve() first, or will there
 always be that much memory available to an array?

   byte[] buffer;
   buffer.reserve(100);
   foreach(byte b; dataSource) buffer ~= b;

Yes, you should always reserve if you know how much data is coming.  If  
you do not here is what happens:

Upon adding the first byte, a block of 16 bytes is allocated
Upon adding the 16th byte (there is one byte for padding), a *new* block  
of 32 bytes is allocated, and the 15 previous bytes are copied to the  
32-byte block.  The original 16 byte block is left allocated because there  
could be other aliases to the data.
Upon adding the 32nd byte, a block of 64 bytes is allocated, rinse and  
repeat.
Upon adding the 64th byte, a block of 128 bytes is allocated, same deal.

Then the block will hold 100 bytes.

If you reserve 100 bytes, then a block of 128 bytes is allocated, and your  
data all goes in there.

OT, I assume you realize that reading data in this way is horribly  
inefficient :)  You should read a block at once if possible.

-Steve

"Lars T. Kyllingstad" <public kyllingen.NOSPAMnet> writes:

On Wed, 14 Jul 2010 10:35:19 -0400, Steven Schveighoffer wrote:

 On Wed, 14 Jul 2010 06:01:10 -0400, Lars T. Kyllingstad
 <public kyllingen.nospamnet> wrote:
 
 Is object.reserve() only useful for large arrays, or should I always
 use it if I intend to append to an array?

 Let's say I want to read some data, and I expect there to be ~100 bytes
 of data.  Is there any point in using reserve() first, or will there
 always be that much memory available to an array?

   byte[] buffer;
   buffer.reserve(100);
   foreach(byte b; dataSource) buffer ~= b;

 
 Yes, you should always reserve if you know how much data is coming.  If
 you do not here is what happens:
 
 Upon adding the first byte, a block of 16 bytes is allocated Upon adding
 the 16th byte (there is one byte for padding), a *new* block of 32 bytes
 is allocated, and the 15 previous bytes are copied to the 32-byte block.
  The original 16 byte block is left allocated because there could be
 other aliases to the data.
 Upon adding the 32nd byte, a block of 64 bytes is allocated, rinse and
 repeat.
 Upon adding the 64th byte, a block of 128 bytes is allocated, same deal.
 
 Then the block will hold 100 bytes.
 
 If you reserve 100 bytes, then a block of 128 bytes is allocated, and
 your data all goes in there.
 
 OT, I assume you realize that reading data in this way is horribly
 inefficient :)  You should read a block at once if possible.


Yeah, that was just an artificial example. :)  The actual use case was 
building a string from substrings.

Thanks!

-Lars